1. Field of the Invention
The present invention is generally in the field of electrical circuits. More particularly, the invention is in the field of transmission line termination circuits.
2. Background Art
Transmitter modules, which can include line drivers, can be utilized in set-top boxes, cable modems, routers, computer interfaces, and other electronic devices to drive transmission lines, such as Ethernet transmission lines. For example, a transmitter module including a line driver can be utilized in an Ethernet interface in a computer. To prevent return loss and signal distortion, for example, a transmitter module or a line driver in the transmitter module is typically required to provide an output impedance that matches the characteristic impedance of the transmission line. For example, a line driver in an Ethernet application can be required to provide a 50.0 ohm output impedance to drive a 50.0 ohm load via a 50.0 ohm Ethernet cable coupled to an Ethernet port. Conventionally, a transmitter module can be passively terminated by utilizing one or more termination resistors to provide a required output impedance. However, the conventional approach to providing a termination resistance for transmitter modules or line drivers in the transmitter modules can have undesirably consequences.
Conventionally, for example, a line driver, such as a current-based line driver, with differential outputs in a transmitter module can be passively terminated by utilizing two termination resistors coupled across the positive and negative outputs of the line driver. In an Ethernet application, for example, each of the termination resistors can have a resistance of approximately 50.0 ohms to provide approximately 100.0 ohms across the differential positive and negative outputs of the line driver. However, when the line driver is transmitting data over a transmission line, a large amount of the current generated by the line driver can flow through the termination resistors and remain on-chip, thereby undesirably increasing on-chip current consumption. As a result, heat generated by the current flowing through the termination resistors can increase packaging cost required to handle the additional heat. Also, the line driver can require a larger layout area to handle the additional current required, which can undesirably increase manufacturing cost.